This invention relates to suction plants for sucking suspendible material by means of a suction nozzle movable along three relatively perpendicular axes, one of said axes being a vertical axis, while the other two axes lie in the horizontal plane. The invention more specifically concerns a development and improvement of a control system of the type covered by U.S. Pat. No. 4,037,335 and U.S. Pat. No. 4,108,499.